Bed rest secondary to illness, surgery or injury often has serious consequences for the older adult, including diminished physical function and compromised independence. Older adults may lose the capacity to perform basic activities of daily living and lose their ability to ambulate short distances. In addition, older adults may not recover function following a period of bed rest, resulting in permanent loss of function and possible nursing home placement. Rehabilitation has been found to be more effective following bed rest than no intervention at all but rarely do patients regain former levels of function. New strategies to minimize loss of function during bed rest and strategies to optimize return of physical function are needed. One possible strategy to attenuate loss of function and physical decline is to provide pre-habilitation services prior to a period of bed rest. It is the intent of this study to examine the effectiveness of pre-habilitation exercises, compared to traditional rehabilitation, using an animal model of bed rest. Changes in walking ability, muscle function and stiffness, and bone mass and strength will be examined in young and old animals receiving pre-habilitation exercise before simulated bed rest. These will be compared to gains made in young and old animals receiving rehabilitation exercise after simulated bed rest. If preliminary findings are confirmed, it would provide support for appropriate human studies, and for expansion of habilitation services to older adults to include both pre-habilitation and rehabilitation.